It's hard to know exactly what you're experiencing. The castle will spawn on the surface once detected regardless of if Rasputin is alive or dead. As of Sunday, you still couldn't detect it unless you were underwater (again, I've never tested at periscope depth) at somewhere around 4k. It's possible someone else spotted the castle and caused it to spawn. It's possible you bobbed underwater briefly at the correct distance and caused it to spawn.

The castle can be spotted in 2 ways. Killing Rasputin causes the castle to automatically spawn on the surface. The other way is to get within ~4km and spot it underwater. It cannot be spotted on the surface prior to Rasputin's death. I'm not sure if periscope depth is sufficient but I always do a full dive around 5km to minimize any incoming damage.

The PTS hard mode had a reduced timer on discovering the black fortress (or castle or whatever it's called) and more towers in awkward positions to find. I think Rasputin also had more health and healed faster. As long as someone was quick on getting fortress/castle, it didn't seem too bad.

Let me see if I can help clarify this for you. Your confusion is that fusing is based on effective armor, not listed armor. That is, a 20mm plate is only 20mm thick when entered head on (at 90 degrees). When entered at an angle, it has greater thickness as more armor has to be passed through. At very oblique angles, even the thinnest armor has large effective armor. This is why overmatch exists, otherwise all ships could sit bow on and tank anything. You can look at wowsft or tanks.gg to see examples of effective armor vs listed armor.

Which DD is immune to radar? Just because Khab fights at 15k and doesn't conceal doesn't make it immune to radar. Plenty of times I've radared a Khab that went stealth to heal. All dds are affected by radar. Many BBs are not affected by this change.
The goal, as stated by WG, is to get rid of 0 damage pens. Just because you've assigned a separate goal doesn't make that the goal.
If this is the goal, its a dumb way to implement it. The only way this make sense is if you believe that Colorados are outperforming Scharnhorsts. If the goal is to reduce BB survivability, there are about a dozen more intuitive, effective ways to do it. This is why this clearly cannot be the goal.
Just to be clear, I'm not arguing against reducing BB survivability, I'm arguing that this is a dumb change because its counter-intuitive and only affects a subset of ships that an average user wont understand while not actually achieving the desired effect.

This change is actually very different than radar. Radar is a mechanism that globally effects DDs in an effort to balance them. We can debate if its useful or not, but that is how it works. This change only effects a subset of BBs and some cruisers. If this was like radar, it would effect all BBs. The goal here is clearly to reduce a counter-intuitive event (0 damage pens) but has introduced another counter-intuitive mechanic, pens to ships that have torpedo bulges. The only way this makes sense from a balancing perspective is if all the ships with exposed torpedo bulges are over-performing. Is Alabama really that much better than Tirpitz? Are Nagato's crushing KGVs? Are French cruisers good?

Regarding the second part, Spruance wanted to engaged Yamato with battleships but Mitscher launched his aircraft for a full attack, and then told Spruance that he did it basically forcing Spruance's hand.
As for Yamato's weakness, I think its fine. Yamato is still the best BB at holding a position if her flank are secure.

I wouldn't be concerned. Most people generally only give positive karma if you get really high XP or are really nice in chat. Most people only give negative karma if you're a [edited] in chat (although some people give bad karma for "bad play"). Basically, just play the game and don't be a bastard and let karma sort it self.

This is actually something relatively easy to show both using probability theory, simulation and empirical results. As simulation is the easiest to explain, I'll use that. There are other posts all along the internet that can explain it either through probability and through empirical results. Let us a setup a simple 12v12 game where each side tries to kill the other side based on their skill. Their skill is the ability to kill another player. Now, we'll make the game somewhat difficult. That is, per round, a player can kill another player 5% of the time (probably still higher than WoWs). Alright, if we set all players to the same skill, we find that the average number of players on the remaining team is 6.17937 with a standard deviation of 2.5531904729 over 100000 games. If we adjust this so that player skill is +/- 10% (relative here) randomly sampled (gaussian) , we get 6.15795 with a standard deviation 2.5413394857. If we adjust the game so that +/- 40% is in play (this would be hugely lopsided as the worst players would be at 20% relative skill of the best players), we get 6.44719 and 2.60015743986. That is, even with ridiculous disparity in potential skill, we still find that our average outcome and std deviation are nearly the same. This is due to the fact that players are randomly sampled. If you always matched bad players against good players (which WoWs is not doing), you would have different results.
But let us also examine just the equal skill game. We see that we have, on average, 6 players remaining with a standard deviation 2.5. You can expect nearly 8.5 players to remain on the winning team as often as you'd expect 3.5 players to remain. Now, this does assume human skill is distributed gaussian but even with perfectly matched teams, we'll get commonly lopsided victories.
LWM also covered this from a theoretical aspect in this thread.

What QoL will it improve? Will you go on losing streaks? Yes, with almost the exact same distribution and likelihood. Will you experience lopsided matches? Yes, with almost the exact same distribution and likelihood. Less Salt? Unlikely based on LoL and Destiny. What evidence do we have that players are leaving based on the MM rather than gameplay? Little to none. So, how exactly will we experience a huge QoL boost?

Agreed, and this is really the core of my problem. Until I can clearly see the benefits and detriments of a SBMM system, its unclear to me that implementing one in WoWs will provide any advantage. I'm not against a SBMM system, but it just isn't clear that it provides a benefit for the "problems" people currently have in WoWs. And I agree, if someone would study this problem, that would help a lot with a debate like this. Unfortunately, this issue seems very complex, especially when you consider 12v12 team games vs 5v5 team games, vs 1v1 games, vs 100 player FFA.
Anyway, I think I've made my point pretty clearly. If, at some point in the future, the benefits of a SBMM are made clear, I'll definitely be on board. Until then, I remain skeptical.

A) There is no evidence that, on average, their is such a skill disparity as you describe.
B) Sports are a bad example as they do not use SBMM. The Cleveland Browns and New England Patriots exist in the same league where one is significantly better than the other (this is an example, not a commentary on those teams as of today). Further, its not clear that players want SBMM. It seems that players are very happy forming super teams.
C) As I said, there is no evidence that SBMM improves player happiness.
I agree, that if player disparity was so large, you would want to segregate the groups. But, as I said, this is just completely unclear. Just to make this point clearer based on your example, 11 Kindergartners and 1 semi pro player will not beat 12 high schoolers, ever. That disparity is huge. A 38% player in WoWs will win 38% of the time. The disparity is much much smaller, even with this "horrible" player.